The invention relates to automatic frequency control of oscillators and in particular, to centering the free running frequency of Injection Locked Oscillators (xe2x80x9cILOxe2x80x9d) during normal operation.
The use of Injection Locked Oscillators has received widespread attention in communication circuits and in frequency synthesis applications. For example, its advantages with respect to locking speed and selectivity are covered in articles by Adler, R. entitled xe2x80x9cA Study of Locking Phenomena in Oscillators,xe2x80x9d Proceedings of the I.R.E, vol. 34, pp. 351-357, (1946) (hereinafter referred to as the Adler paper); by Uzunoglu, V. and White, M. H. entitled xe2x80x9cThe Synchronous Oscillator: A Synchronization and Tracking Network,xe2x80x9d IEEE Journal of Solid-State Circuits, vol. SC-20; (6), pp. 1214-1225, (1985); and by Armand, M. entitled xe2x80x9cOn the Output Spectrum of Unlocked Driven Oscillators,xe2x80x9d Proceedings of the IEEE (Letters), vol. 57, pp. 798-799, (1969) (hereinafter referred to as the Armand paper).
The ILO locks on a signal source if the source is within the frequency lock range of the ILO. Under this lock condition, the ILO generates a signal with a frequency that is identical to that of the driving signal with a transfer phase shift and it provides effective filtering of lower level spurious frequency components that might be present at the driving input to the ILO. The early work as described in Adler""s ILO paper provides the transfer phase relationship and the lock range as a function of the ILO resonant circuit Q and the input drive level. In the lock range, the output amplitude is fixed and equal to oscillator level wherein the output phase with respect to the nominal phase at the center of the lock range is given by an arcsine function: xcex8=arcsin(f/a), wherein f is the frequency offset from the center free running frequency of the ILO and a is the frequency lock range which is proportional to the drive level amplitude.
The transfer characteristics outside the lock range can be obtained from the first term of the Fourier analysis given in Armand""s paper for the spectrum of an unlocked driven oscillator.
Following the Armand paper we have for |f| greater than a the out-of-lock transfer response as a function of the frequency offset with respect to the free running ILO frequency is proportional to                               i          ⁡                      (                          f              a                        )                          ⁢                  (                      1            -                                          1                -                                                      (                                          a                      f                                        )                                    2                                                              )                                    (        1        )            
Therefore, in our model, the composite ILO transfer response is given by Equation (2).                               S          ⁡                      (                          f              ,              a                        )                          =                  {                                                                      e                                                            i                      ⁢                      arcsin                                        ⁡                                          (                                              f                        /                        a                                            )                                                                                                                                        for                    ⁢                                          xe2x80x83                                        ⁢                                          "LeftBracketingBar"                      f                      "RightBracketingBar"                                                        ≤                  a                                                                                                                          a                    ·                                          V                      a                                                        +                                                            i                      ⁡                                              (                                                  f                          a                                                )                                                              ⁢                                          (                                              1                        -                                                                              1                            -                                                                                          (                                                                  a                                  f                                                                )                                                            2                                                                                                                          )                                                                                                                                        for                    ⁢                                          xe2x80x83                                        ⁢                                          "LeftBracketingBar"                      f                      "RightBracketingBar"                                                         greater than                   a                                                              )                                    (        2        )            
The term axc2x7Va is due to drive level leakage into the ILO output structure and is proportional to input level through a and the complex coupling coefficient Va, which typically is in the order of xe2x88x920.1 (xe2x88x9220 dB and out-of phase). This composite transfer response is shown in FIG. 1 for such a typical Va value and in FIG. 2 for Va of the same magnitude but with a phase of xe2x88x9245xc2x0.
As noted above, FIG. 1 shows the Injection locked oscillator (ILO) selectivity characteristics. This phase-magnitude transfer function represents the response of the ILO to a constant level drive signal and is a function of the normalized frequency centered about the free running frequency of the ILO. FIG. 2 shows that ILO response can often be skewed by phase offset of the driver leakage term. Here, the phase coupling is at xe2x88x9245 degrees. Alternatively, phase coupling of xe2x88x92135 degrees will produce an opposite effect of accentuating lower frequencies at the expense of higher frequencies.
The value of employing the ILO in signal synthesis applications can be realized only if its center frequency can be maintained so that it is within the lock range from the frequency of a desired signal source that one wishes to reproduce with significant rejection of undesired spectral components. Prior art methods of achieving that goal have been described by Joynson et al. in U.S. Pat. No. 5,107,272 issued on Apr. 21, 1992. By utilizing the special phase transfer relationship, Joynson et al. vary the drive frequency to track the free running frequency of the ILO so as to maintain lock. However, it is often desired to use a synthesized drive signal of a predetermined and non-variable frequency and still be able to reliably maintain ILO lock without unduly broadening the lock range. This capability will permit the accommodation of thermal drifts while still maintaining a superior selectivity due to a narrower lock range.